B767 Captain reported refusing to operate the aircraft due to an applied MEL that compromised safety during ETOPS operations.

Narrative

A seemingly simple deferral of the Left Anti-Ice Cowl Valve locked in the open position creates some significant safety concerns in the unlikely; but not unheard of; event of loss of all AC power. The HMG (Hydraulic Motor Generator) is not available in this scenario due to the way that the pneumatic system is to be operated with the cowl valve locked open. So if a complete loss of AC power were to occur; then the following checklist item XX WARNING from the (YYY) AC BUS OFF - L (R) Non-Normal checklist (page X of the 757-767 FM) would apply: 'XX WARNING: If the HMG fails or is inoperative; flight beyond 30 minutes will result in complete loss of electrical power and the inability to lower gear and flaps.' So in analyzing the flight on MM/DD/YYYY; 138 minute ETOPS was planned using ZZZZ1 and ZZZZ2 as the ETOPS alternates. (Actual condition times for the diversion were even greater than the 138 minutes planning assumption due to forecast winds.) So the possibility of losing all electrical power is almost a certainty if the loss of AC power occurs at or near the Critical Point. And this could even be an issue when only 60 minutes from an Intermediate Airport.So in considering those possibilities with consultation with Dispatch; I determined that the prudent and safe course of action was to refuse this aircraft; nose number xxxx. Safety is our number one principle; and I acted with that in mind. Also; I coordinated with Maintenance using the code as outlined in pilot bulletin; unable to operate procedure; so that a 'UNABLE TO OPERATE' log item was entered into the aircraft history. Lastly; I just could not see how I could defend taking the aircraft if an unlikely; but not unheard of; loss of all AC power were to occur. And if I did end up in a situation as described in the warning; would I even be able to safely recover the aircraft so that I could defend the decision? That is a question I did not want to have to answer.